Enlarge / This stovepipe tornado formed under an intense rotating wall cloud near Keota, Iowa, on March 31, 2023.
One muggy day in July 1986, a news helicopter was recording footage of a festival in Minneapolis when the pilot and photographer glimpsed a tornado over nearby Brooklyn Park. They moved toward it, filming the powerful twister for 25 minutes, mesmerizing viewers watching it live on TV.
Watching as the helicopter hovered within maybe a half-mile of the twister was Robin Tanamachi, who was a kid growing up in Minneapolis at the time. “We were seeing all this really beautiful interior vortex structure,” she says. “I was just absolutely hooked on that, and I know I was not the only one.” Today, Tanamachi is a research meteorologist at Purdue University in West Lafayette, Indiana, and one of many researchers delving into twisters’ mysteries, searching for details about their formation that may bolster future forecasts.
Tornadoes can be elusive research subjects. Through chasing storms and using computer simulations, scientists have worked out the basic ingredients needed to spin up a twister, but two crucial questions continue to vex them: Why do some thunderstorms form tornadoes while others don’t? And how exactly do tornadoes get their spin?
Despite the logistically and scientifically challenging nature of the work, scientists are motivated to keep trying: Tornadoes can kill dozens to hundreds of people in the United States every year and cause billions of dollars in damage. Now researchers are chasing the killer storms that spawn tornadoes with cutting-edge technology, flying drones into the storms and harnessing more computing power than ever to simulate them in search of answers.
“Today, we’re simulating the atmosphere with unprecedented spatial resolution. We’re observing storms with unprecedented temporal and spatial resolution,” says atmospheric scientist Howie Bluestein of the University of Oklahoma in Norman. “But there’s still a lot of problems and a lot of things that need to be solved.”
Scientists may be turning up new clues to tornado formation by studying what’s happening in the atmosphere around them and on the ground below them, and by comparing what they find in the field with new, higher-resolution models of the thunderstorms that generate them. Even as they chase these new leads, researchers are also trying to understand how climate change may affect when and where tornadoes form.
Since scientists began studying tornadoes in earnest in the mid-20th century, they’ve put together a pretty good outline of the steps required to generate a twister. Most destructive tornadoes are spawned by supercell thunderstorms—giants that typically have a very tall cloud that widens into an anvil shape at the top. Supercells are characterized by a kilometers-wide rotating updraft called a mesocyclone that can last for hours. That rotation comes from wind shear, which sets wind nearer to the ground spinning horizontally like a spiraling football. These winds then become vertically oriented within an updraft like a spinning top.
A couple of things need to happen for a supercell to become tornadic: First, the giant mesocyclone at the heart of the storm needs to get air rotating closer to the ground. Then this vortex needs to be stretched upward. Stretching tightens the twister’s footprint, speeding its rotation, similar to what happens when figure skaters pull in their arms during a spin.
The first clues to the physics of tornadoes came from secondhand information and damage reports, as scientists tried to figure out what sorts of winds could blow down a barn or pluck a chicken, says Richard Rotunno, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado, and the author of an overview of the fluid dynamics of tornadoes in the 2013 Annual Review of Fluid Mechanics.
The construction of the Interstate Highway System in the 1950s created a grid across the flat Great Plains that allowed enterprising scientists to get out in front of storms and sometimes directly observe tornadoes. A big advance came with the development of Doppler radar for meteorology. By emitting pulses of energy and detecting the reflected signal, the technology captures information about wind and precipitation. Radar allowed the detection of mesocyclones, which became the basis for tornado forecasts and a boon for chasers, who would stop at payphones periodically to call the lab for the latest radar intel.
Source : Ars Technica